Many semiconductor materials (which include certain photoconductive materials) are extremely difficult to form into three-dimensional (3-D) coils. For making devices, most semiconductor materials are formed as respective layers (two-dimensional or 2-D structures) using any of various surficial techniques such as chemical vapor deposition, physical vapor deposition, epitaxy, sputtering, or vacuum deposition. Whereas these layer-forming techniques are effective for forming substantially 2-D shapes, general success with forming 3-D structures of such materials by these techniques on a size scale greater than MEMS has been elusive. As used herein, a “3-D” or “3-dimensional” structure has respective dimensions in all the x, y, and z directions that are greater than a layer thickness formable by conventional semiconductor layer-forming techniques. For example, a coil made from a thin layer of semiconductor or metal material is a “3-D” structure if it has been formed into a structure having respective dimensions (in each of the x, y, and z directions greater than the thin-layer thickness.
In U.S. Patent Publication No. 2007-0007844 A1, a 3-D semiconductor-metal coil is configured as a metal coil coated with a photoconductive material. The metal coil, made of metal wire, is made first, followed by coating the metal coil with a semiconductor material, such as a photoconductor. Unfortunately, it is difficult to achieve satisfactory adhesion of the photoconductive material to the metal of the coil, even in instances in which the semiconductor material is applied as a slurry to the metal coil. Also, since these coils lack any physical support, they are too fragile for practical use.